First choice was easy: plot sourcebooks (with a strong preference towards factual ones). Books like the Jihad series are a nice change of pace, but I can only accept them if they are accompanied by some cold, hard facts. Book after book filled with nothing but sidebars? Not so enjoyable. More WoR, less JHS.
As for the rest:
TRO's: Did not get my vote. As others have pointed out, I feel like we are very nearly over-saturated with these. Yes, I will buy a new one without hesitation, but there have been so many as of late (including XTRO's) that there is just not enough time to familiarize oneself with the new units and how they fit into the universe before the next one is released and this process begins anew. It would be nice if I (and the RS writers) had time to properly catch up.
FM's: Did not get my vote because, unless a new, fairly stable faction has been created (GB Dominion, Raven Alliance, RotS, etc.), I feel these are covered fairly well by the Field Reports and Objectives series.
Era Reports: While they provide a nice summary for those who may have passed over/missed out on an era, this is about what suits my fancy, so no vote from me.
Thus, my second choice is Historicals. No reason we can't go both forward and backward in time. ;)
...Oh, and if I could have a third choice, it would be for Other - Interstellar Operations. O:-)